Taco shells are U-shaped fried tortillas to be filled with a variety of foodstuffs and eaten. While they have been made for many, many years there is still no convenient way, outside of a production environment, to take a soft, pliable tortilla and turn it into fried U-shaped hard shell taco. If it is formed in a U before being fried, a tall, narrow very deep container of hot oil might be used. This type of container, including a way to keep its oil uniformly hot, is not a standard household item. If only a shallow layer of hot oil is available, then the U-shaped tortilla would need to be set in the oil twice, once per side.
Alternatively if the tortilla is fried flat then it will become stiff and brittle throughout and not amenable to be bent into a U-shape unless it is manipulated in a complex manner while frying. However it is produced, the result is a U-shaped brittle taco shell that is likely to crack when foodstuffs are added or when the first bite is taken.